A healthy, rhythmic heart beats regularly to pump blood throughout the body. When the heart is experiencing an arrhythmia it does not pump blood as effectively. The most severe of all arrhythmias is ventricular fibrillation (V-fib). V-fib is a condition in which the heart experiences circus movement; that is, it contracts in an unsynchronized, discordant manner. During V-fib, the heart virtually ceases to pump blood and death is imminent unless a normal heart beat is restored.
A defibrillator is a device that provides energy in the form of a large electrical pulse or pulses to the heart for the purpose of interrupting the circus rhythm. A successful defibrillation interrupts the fibrillation and restores a more normal, life sustaining rhythm. A defibrillator may be either external or implantable. A defibrillator has electrodes that are placed on or in the body which provide an interface through which defibrillation energy is delivered to the body.
Some patients have a particularly high susceptibility to V-fib. For such patients, it is desirable to implant a defibrillator that has the potential to continuously sense the rhythm of the heart and deliver the defibrillating energy whenever it is needed. Such continuous sensing obviates the need for a patient to be continually monitored by medical personnel.
Defibrillators that are implanted within the body are necessarily relatively small in order to fit within the restricted space available in a human body. This space restriction imposes limits on the amount of electrical energy that may be stored in the defibrillator that is available for generating defibrillation pulses. Such electrical energy is typically stored in batteries positioned within the defibrillator. So that the implantable defibrillator may operate a long time before its battery is depleted, it is desirable to minimize the energy expenditure required for each defibrillation pulse. One way of reducing the defibrillation energy storage requirements is to deliver the energy in a very efficient manner.
Efficient delivery of defibrillation pulses may be accomplished by placing the defibrillating electrodes close to the heart. This is commonly achieved (as in U.S. Pat. Nos. 3,942,536, 4,161,952 and 4,355,646) by conducting the energy from the defibrillator to the heart through flexible, insulated leads that are generally connected to electrodes that are placed on, or very near to the heart.
To further minimize the energy required to defibrillate the heart, prior art defibrillation systems have used a variety of electrode shapes, materials, locations and even electrical wave forms and pulse trains. The electrodes that generally permit defibrillation with the lowest electrical energy expenditure have typically been placed directly on or very close to the heart. Such electrodes are referred to as patch electrodes. These electrodes also have relatively large surface areas, so that they cover a good deal of the surface of the ventricles of the heart.
Since the electrical current associated with a defibrillation pulse is conducted out of one electrode and back into the other, the locations where the electrodes are placed on the heart have a significant influence on the energy expenditure needed for defibrillation. Efficient delivery of defibrillating energy is associated with a fairly uniform energy density (energy per volume area) distribution throughout the ventricles. Uniform energy density through the ventricles allows the entire ventricular mass to be simultaneously depolarized without expending excess energy. Excessive energy is not only inefficient, it also may actually cause irreversible damage to the heart, for example by burning heart tissue. The inefficient use of energy shortens the expected life of the implantable defibrillator, and may even result in defibrillation thresholds so high that the heart can not be defibrillated by the implanted defibrillator. A defibrillation threshold is the energy level required to effectuate defibrillation of the heart.
Accordingly, access to the heart is necessary to facilitate the most advantageous placement of the defibrillating electrodes in a manner which promotes uniform energy distribution through the ventricles and which minimizes the energy expenditure required to effectuate defibrillation. Typically, attachment of defibrillation electrodes to the heart requires an open chest procedure; either via a median sternotomy; intercostal approach; or, in a more limited procedure, a sub-xiphoid approach. All of these procedures involve major surgeries which are painful and dangerous for the patient.
Alternative methods of placing defibrillation electrodes are less invasive. Such less intrusive methods employ transvenously placed endocardial electrodes and/or subcutaneous electrodes. Transvenously placed endocardial electrodes are placed inside the heart by threading them through a large vein, such as the vena cava. Subcutaneous electrodes are placed under the skin somewhere in the upper thorax in locations relatively distant from the heart. Electrodes placed solely by these means generally result in much higher defibrillation thresholds than exhibited by patch electrodes sutured directly to the heart.
An automatic implantable defibrillation system continually monitors the heart rhythm for V-fib or other lethal arrhythmias. When the defibrillation system recognizes a lethal arrhythmia, where the heart fails to pump blood to the body, the defibrillator delivers one or more electrical energy pulses to the heart to re-establish a life sustaining rhythm. An automatic defibrillation system obviates the need for a patient to wait for the arrival of trained medical personnel to diagnose and treat a lethal arrhythmia.
An object of many defibrillation lead implant procedures is to not only place the leads effectively, but to do so with a minimum of trauma to the patient. Minimization of trauma should be associated with a concomitant reduction in morbidity and mortality for the patients undergoing such procedures. Rather than have a patient undergo an open chest procedure (thoracotomy), even a limited one, it is desirable to employ a procedure entailing less risk and discomfort for the patient. Therefore, there is a need for methods and equipment that enable the effective placement of defibrillation electrodes on or near the heart in a manner which minimizes trauma, risk, and discomfort for the patient.
The heart resides within a thin, lubricous, protective sac known as the pericardium which is a membranous sac that encloses the heart. It consists of an outer layer of dense fibrous tissue and an inner serous layer, termed the epicardium, which directly surrounds the heart. Throughout the description and claims herein, the phrase "within the pericardium" or "within the pericardial space" refers to any of the body tissues or fluid found inside the dense outer surface of the heart, but not including the interior of the heart. The narrow space between the pericardium and the heart is filled with a thin layer of pericardial fluid and is referred to as the pericardial space.
It is generally known that the pericardial space represents a propitious location for defibrillation electrode placement. For example, U.S. Pat. No. 4,991,578 describes a system for positioning a defibrillation electrode within the pericardial space of a mammal The system described in the '578 patent includes means for distending the pericardium from the heart by injecting a small volume of fluid into the pericardium, e.g., from a location inside the heart. A needle having a lumen therethrough is inserted from a sub-xiphoid or other percutaneous position into the body until a tip thereof punctures the distended pericardium at a selected location. A guidewire is inserted into the pericardium through the lumen of the needle, and while the guidewire remains in the pericardial space, the needle is removed. A sheath is introduced over the guidewire, with the aid of a dilator, and inserted into the tissue until the end of the sheath is positioned within the pericardium. The defibrillation lead, with its electrode in a retracted position, is inserted through the sheath until the electrode is likewise positioned within the pericardium, whereupon the electrode is deployed in order to make contact with a large area of tissue within the pericardium.
However, in some patients, access to the pericardial space is not readily achievable, due to the presence of proliferative adhesions between the pericardium and the heart. Application of the '578 system would not benefit such patients because injection of fluid between the pericardium and the heart would not result in distension of the pericardium. The system described in the '578 patent disadvantageously requires perforation of the heart wall. Such perforation can result in severe bleeding, requiring emergency open thoracic surgery.
Using only currently available techniques, pericardial access on defibrillation patients can be a dangerous proposition. The pericardium is both very thin and close to the heart. In most patients it is difficult to cut or puncture the pericardium without also inadvertently cutting or perforating the myocardium (the heart muscle) or a coronary vessel.
To place a sheath in a vessel by the technique known in the art as a percutaneous stick, a needle is placed into the desired vessel, a guidewire is placed through the needle, and then the needle is removed. Then a dilator/sheath passes over the guidewire and into the vessel. A similar technique could be adapted to gain access to the pericardium for the purpose of lead placement. There are at least two potential impediments to direct placement of a dilator/sheath introducer set into the pericardium: (1) relatively loose attachment of the pericardium with respect to the chest wall may make passage of a dilator through the very tough pericardium difficult; and (2) rapid, inadvertent introduction of the dilator into the pericardium could easily impale the myocardium or a coronary vessel. Thus, there is a need for equipment and methods to safely enter the pericardial space with an introducer sheath.
The site at which the surgical instruments are brought into the proximity of the heart is very important. All actions in this vicinity must be careful and deliberate. Because the heart is in constant motion, it is very difficult to avoid damaging the heart. There are no instruments known in the art that permit precisely metered motion relative to the moving surface of the heart. If a sharp instrument were to impinge upon a beating heart, an unintended perforation of a coronary artery or myocardial wall could occur. Such perforation would result in massive bleeding, endangering the life of the patient, and probably necessitating emergency open chest surgery.
There are no safe and effective percutaneous methods and instruments known in the art for placing defibrillation leads either in the pericardial space or extrapericardially (outside the pericardium) with anchoring close to the heart. There are no defibrillation leads known in the art that are ideally suited to being placed through percutaneous introducers and situated in the pericardium or on the pericardium with anchoring close to the heart. Therefore, there is great need for a method and system for implanting defibrillation leads to the surface of the heart or pericardium that provides good anchoring and that does not require perforation of the heart tissue and exposure of the patient to the risk of severe bleeding and possibly death.